A free electron laser requires a beam of relativistic electrons to be superimposed with an intense laser beam within a static, periodic magnetic field. Energy is exchanged between the electrons and the laser radiation field to produce coherent radiation or amplification of the laser beam.
The radiation energy density of a high power free electron laser can be on the order of tens of megawatts/cm.sup.2. Such high flux densities will damage a mirror surface.
If a wiggler with two spherical mirrors is used in a conventional stable resonator and diffractive spreading of the beam is relied on for flux reduction, then mirror spacings of several hundred meters would be required to obtain acceptable irradiance levels for even the most advanced high power optics, such as actively cooled silicon.
If such long resonators are used, then alignment of the beam to keep it coaxial with the electron beam becomes extremely critical, and may be the limiting parameter for the device.
If the optical elements nearest the wiggler are at grazing angle of incidence then the overall length of the resonator can be considerably reduced. At grazing incidence the area of the optical mode can be considerably larger than at normal incidence. A ring configuration (rather than a standing wave configuration) further reduces the resonator length by approximately a factor of two since one way flux is incident on the grazing optical elements instead of two-way flux. Since the free-electron laser gain medium is unidirectional, the reverse mode is absent and no additional means are required for its suppression.
In stable resonators, output is taken through a partially transmitting element which cannot be actively cooled, and therefore can be easily distorted or even destroyed by high flux densities. An unstable resonator permits convenient output coupling around the edges of a scraper element which can be made out of metal, or other high conductivity materials like silicon and can be efficiently cooled with coolant flowing through the material. In a stable resonator an alternative output coupling scheme may be a grating rhomb arrangement. The grating rhomb is a costly element, susceptible to damage and it limits the tuning range of the otherwise continuously tuneable free-electron laser. No such restrictions are imposed by an unstable resonator.
The extraction efficiency of the free electron laser is an important performance parameter. Because of the limitations discussed above, a free electron laser with a stable resonator is likely to be less efficient than the one using an unstable resonator.